1. Field of the Invention
The present invention relates to thermoplastic molding materials having dimensional stability under heat and stress cracking resistance and exhibiting a high degree of transparency.
2. Discussion of the Background
DE-A 12 31 013 describes a method for producing copolymers which are composed of 1 to 50 wt % of an .alpha.-alkylstyrene and 99 to 50 wt % of an alkylmethacrylate, optionally containing secondary amounts of other polymerizable compounds, wherein the copolymers contain 0.1 to 20 wt % maleic anhydride and/or methacrylic acid. Molding materials made from these copolymers display little tendency to form cracks when they are alternately exposed to hot and cold water.
DE-A 23 41 123 discloses thermoplastic molding materials which are composed of 50 to 99 wt % styrene or (C.sub.1 -C.sub.8 -alkyl)methacrylate, as well as 1 to 50 wt % of an .alpha.-substituted acrylic acid, such as methacrylic acid, wherein at least 5 wt % of the acid must be present in the form of acid anhydride. The transfer of a part of the carboxyl groups into anhydride groups is accomplished with the help of a thermal treatment of the molding material at a temperature above the Vicat softening point.
JP-A 83 125 712 describes molding materials which are composed of copolymers from 50-98 wt % methylmethacrylate, 1 to 25 wt % styrene, and 1 to 25 wt % methacrylic acid with a high degree of thermal dimensional stability and a high degree of stability against thermal decomposition.
EP-A 273 397 describes substrates for optical elements which are composed of copolymers from methylmethacrylate, aromatic vinyl compounds, an unsaturated carboxylic acid, and glutaric acid anhydride units, which have good thermal dimensional stability and a high degree of transparency.
U.S. Pat. No. 2,851,448 discloses transparent ter-polymers which are produced by chamber polymerization of monomer mixtures of vinyl aromatic compounds, alkylmethacrylates, and unsaturated carboxylic acids, such as acrylic or methacrylic acid, in specified compositions.
EP-A 264 508 describes a process for the production of copolymers having high degrees of thermal dimensional stability which comprises a secondary thermal treatment of the copolymers made from vinyl monomers, such as styrene and methylmethacrylate, and from acrylic or methacrylic acid units. During the secondary thermal treatment, (meth)acrylic acid units turn into cyclic anhydride units in the polymer chain.
The copolymers described in the state of the art which contain alkylmethacrylates, vinyl aromatic compounds and unsaturated carboxylic acids are high-grade molding materials with regard to their thermal dimensional stability under heat and their corrosion resistance against different solvents. This is why experts are interested in mixing such copolymers with conventional molding materials in order to improve the properties of such molding materials. If one considers the polymethylmethacrylate molding materials as mixing partners, then the requirement for the technical application of such mixtures is to maintain the high degree of transparency of the polymethylmethacrylate molding material. This means that such mixtures must be compatible with regard to thermodynamic aspects.
As a general rule, polymer mixtures are incompatible. Despite a growing number of specific examples which demonstrate the opposite and which have been found in the past few years, the expert's experience to date is still determined by the following phrase: "Miscibility is the exception, immiscibility is the rule" (ref. Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd. ed., vol. 18, p. 460, J. Wiley, 1982).